2020-01-27, 11:01:32
When it comes to erecting an HF wire antenna the conventional wisdom is “the higher the better”. For many wire antennas that is good advice. But sometimes a very low wire antenna is actually a much better alternative. Let me explain.
I have been using a long wire antenna, both at my home QTH and out in the field (where I prefer to operate in the warmer months) that is approximately 132 feet long and only 12 feet high! It works very well on 20m, 40m and 80m and, theoretically, on 160m too (although I have yet to try it on top band). The low height is not a compromise; it is actually absolutely essential to the operation of the antenna!
I am mostly a CW operator and I needed a NVIS (Near Vertical Incidence Skywave) antenna for my weekly check-in to a 80m CW net in Shelburne, Ontario. I also wanted the same antenna to be usable on my other regular bands of 20m and 40m. A wire that is 132 feet long, erected in a straight line, at a height above ground of only 12 feet checks all the boxes in my needs list. It is a half-wave on 80m and radiates an almost perfectly vertical signal which is reflected by the ionosphere’s F2 layer back down to the ground over a radius of about 400km.
But, how does it work on the higher bands, 20m and 40m? The answer lies in both its length and its low height above ground. On the 40-metre band the wire is a full wavelength long. On the 20-metre band the wire is two wavelengths long. A wire that is at least one wavelength long behaves quite differently to a regular half-wave dipole. A dipole erected high above the ground radiates broadside to the wire. A long wire (sometimes called a “wave antenna”) radiates off the ends of the wire. As the wire is brought lower to the ground the take-off angle decreases. As HF operators know, a low take-off angle is good for DX contacts. An added bonus of a low wire is that it doesn’t pick up as much noise, making weak signals easier to copy.
The downside is that, as the wire is brought lower, ground losses begin to increase rapidly. The U.S. military used wave antennas as low as 1-metre during the Vietnam war. They are very easy to erect and hard for the enemy to spot. Ground losses also limited the range of communication sufficiently to lessen the chance of unwanted enemy signal surveillance.
Hams are generally interested in longer range contacts so ground losses are undesirable. Although even long wires laid directly on the ground actually work (I have QSOd myself with a 148ft long “Grasswire” antenna and only 5 watts of transmitted power), raising the wire up to 12 feet strikes a balance between ground losses and directionality. I have successfully used my home QTH antenna for DX contacts as far away as Europe with less than 100 watts of transmitted power.
It is essential that the wire is run in a straight line for operation on the higher bands. 132 feet of wire just fits within my suburban Owen Sound lot but, of course, length is not a concern when operating from a park in the summer. The wire is fed from one end and that presents a challenge. The feedpoint impedance can be thousands of ohms but that is taken care of by a 49:1 transformer wound on a large ferrite toroid. It is fed by 100 feet of coax and the coax shield acts as a counterpoise. Performance has been outstanding as evidenced by the contacts in my log. NVIS, short range contacts on 80m, mid-range contacts on 40m and long-range DX contacts all from a simple single wire antenna. It is stealthy too which is a bonus when the curiosity of neighbours and Provincial Park rangers is to be avoided!
John Corby, VA3KOT
I have been using a long wire antenna, both at my home QTH and out in the field (where I prefer to operate in the warmer months) that is approximately 132 feet long and only 12 feet high! It works very well on 20m, 40m and 80m and, theoretically, on 160m too (although I have yet to try it on top band). The low height is not a compromise; it is actually absolutely essential to the operation of the antenna!
I am mostly a CW operator and I needed a NVIS (Near Vertical Incidence Skywave) antenna for my weekly check-in to a 80m CW net in Shelburne, Ontario. I also wanted the same antenna to be usable on my other regular bands of 20m and 40m. A wire that is 132 feet long, erected in a straight line, at a height above ground of only 12 feet checks all the boxes in my needs list. It is a half-wave on 80m and radiates an almost perfectly vertical signal which is reflected by the ionosphere’s F2 layer back down to the ground over a radius of about 400km.
But, how does it work on the higher bands, 20m and 40m? The answer lies in both its length and its low height above ground. On the 40-metre band the wire is a full wavelength long. On the 20-metre band the wire is two wavelengths long. A wire that is at least one wavelength long behaves quite differently to a regular half-wave dipole. A dipole erected high above the ground radiates broadside to the wire. A long wire (sometimes called a “wave antenna”) radiates off the ends of the wire. As the wire is brought lower to the ground the take-off angle decreases. As HF operators know, a low take-off angle is good for DX contacts. An added bonus of a low wire is that it doesn’t pick up as much noise, making weak signals easier to copy.
The downside is that, as the wire is brought lower, ground losses begin to increase rapidly. The U.S. military used wave antennas as low as 1-metre during the Vietnam war. They are very easy to erect and hard for the enemy to spot. Ground losses also limited the range of communication sufficiently to lessen the chance of unwanted enemy signal surveillance.
Hams are generally interested in longer range contacts so ground losses are undesirable. Although even long wires laid directly on the ground actually work (I have QSOd myself with a 148ft long “Grasswire” antenna and only 5 watts of transmitted power), raising the wire up to 12 feet strikes a balance between ground losses and directionality. I have successfully used my home QTH antenna for DX contacts as far away as Europe with less than 100 watts of transmitted power.
It is essential that the wire is run in a straight line for operation on the higher bands. 132 feet of wire just fits within my suburban Owen Sound lot but, of course, length is not a concern when operating from a park in the summer. The wire is fed from one end and that presents a challenge. The feedpoint impedance can be thousands of ohms but that is taken care of by a 49:1 transformer wound on a large ferrite toroid. It is fed by 100 feet of coax and the coax shield acts as a counterpoise. Performance has been outstanding as evidenced by the contacts in my log. NVIS, short range contacts on 80m, mid-range contacts on 40m and long-range DX contacts all from a simple single wire antenna. It is stealthy too which is a bonus when the curiosity of neighbours and Provincial Park rangers is to be avoided!
John Corby, VA3KOT
